The present invention relates to a technique for correcting dispersion of a communication semiconductor integrated circuit having a modulation function of a transmission signal and a technique for reducing variation in characteristics due to dispersion in a gain of an amplitude control loop in a communication semiconductor integrated circuit having a phase control loop for phase modulation and the amplitude control loop for amplitude modulation. More particularly, the present invention relates to a technique effectively applicable to a semiconductor integrated circuit for high-frequency signal processing (high-frequency IC) having a modulation/demodulation function provided in a wireless or radio communication apparatus such as, for example, a portable telephone and an electronic component (RF module) using it.
One of the systems for wireless communication apparatuses (mobile communication apparatuses) such as a portable telephone of the prior art is a global system for mobile communication (GSM) adopted in Europe. The GSM system uses a phase modulation method called a Gaussian Minimum Shift Keying (GMSK) method in which a phase of a carrier wave is shifted according to transmission data.
Further, an evolved system of the GSM system to enhance data rates, that is, a system of enhanced data rates for GSM evolution (EDGE) has been proposed, the EDGE system having a dual-mode communication function in which an audio signal is communicated in the GMSK modulation and data is communicated in 3.pi./8 rotating 8-PSK (phase shift keying) modulation. The 8-PSK modulation is a modulation implemented by adding amplitude shift to the carrier phase shift in the GMSK modulation. In comparison with the GMSK modulation in which information of one bit is transmitted per symbol, information of three bits is transmitted per symbol in the 8-PSK modulation. Therefore, communication can be made at a higher transmission rate in the EDGE than in the GSM.
As a method of implementing a modulation method in which each of a phase component and an amplitude component of a transmission signal contains information, there has been known a method in which a signal to be transmitted is divided into a phase component and an amplitude component, a feedback operation is conducted for the components respectively by a phase control loop and an amplitude control loop, and signals resultant from the feedback operations are mixed with each other by an amplifier to output a signal (for example, British Patent Publication GB0212725.6)
In the above-mentioned system which has the phase control loop and the amplitude control loop and can implement wireless communication of the EDGE system, circuits constituting the amplitude control loop have dispersion of the gains due to dispersion in characteristics of constituent elements of the circuits in production and a loop gain is deviated from a design value. Accordingly, there is a possibility that the modulation precision of a transmission waveform (error vector magnitude (EVM)) and the noise suppression degree do not satisfy requirements of specifications.
Concretely, there is a problem in which while the modulation precision is higher and a characteristic called spectral re-growth indicating a degree of waveform distortion becomes better when the frequency bandwidth of the amplitude control loop (a frequency range from a central frequency of the transmission carrier wave to a frequency thereof for which an open loop gain is 0 dB) is wider, an attenuation value of the amplitude control loop becomes smaller for a reception frequency apart by 20 MHz from the central frequency of the transmission carrier wave and a sufficient noise suppression degree cannot be obtained when the bandwidth is wide. The above British Patent Publication discloses the technique for measuring dispersion in gain of the amplitude control loop and calibrating it.
In the amplitude control of the above British Patent Publication, however, a signal fed back to an amplitude comparison circuit for comparing amplitudes between a transmission signal and a feedback signal is taken out from an output of a power amplitude with a coupler provided at a succeeding stage of a high-frequency integrated circuit (IC). Accordingly, it is impossible to measure dispersion in the gain of the amplitude control loop before the system is constructed. To this end, when such measurement and calibration of dispersion are made on the side of a set maker which manufactures the wireless communication apparatus, the burden on the set maker is increased. On the other hand, when the measurement and calibration of dispersion are made on the side of an IC maker which manufactures the high-frequency IC, calibration cannot be made in consideration of dispersion in characteristics of the power amplifiers connected to the succeeding stage of the high-frequency IC and accordingly the precision of the calibration is degraded.
Further, calibration of dispersion in gain of the amplitude control loop in the invention disclosed in the Publication GB0212725.6 is made by taking data for calibrating a gain characteristic to a control signal outputted by a variable gain amplifier connected on the way of the amplitude control loop on the basis of a measured value to eliminate the dispersion and storing the calibration data in a nonvolatile memory in a baseband LSI circuit. Accordingly, the writing work of the calibration data in the nonvolatile memory is required to increase the burden on the set maker. In addition, it is also considered that a circuit for adjusting dispersion of the gain is provided for each circuit having large dispersion of the gain so that the gain is adjusted by the adjusting circuit on the basis of the measured result, although since such a method requires measurement and adjustment processes of dispersion for each chip, a cost of the integrated circuit is increased.